1. Field of Invention
The present invention relates to a process for the preparation of (co)polymers by controlled/"living" atom transfer radical polymerization in an emulsion or suspension, and the emulsions and suspensions prepared thereby.
2. Discussion of the Background
In order to obtain well defined polymers (with respect to characteristics such as predefined molecular weight, narrow molecular weight distribution, well defined end groups, composition and architecture), "living" or controlled chain growth polymerizations are often employed. Webster, O. Science 1991, 251, 887. These polymerizations are special in that chain breaking reactions, such as termination or transfer, are minimized such that their contribution to the overall polymerization is negligible. Such polymerizations are characterized by initiation where one initiator molecule generates, at most, one polymer chain and that all polymer chains grow at nearly the same time. This results in polymers whose number average molecular weight is defined by M.sub.n =MW.sub.m *(.DELTA.[M]/[I].sub.o)+MW.sub.int, where .DELTA.[M] is the concentration of consumed monomer, [I].sub.o is the concentration of added initiator, MW.sub.m and MW.sub.int are the molar masses of the monomer and added initiator, respectively. The molecular weight distributions, defined as MWD=M.sub.w /M.sub.n, are generally narrow, with MWD&lt;1.5.
The first living polymerizations were those based on the ionic polymerization of vinyl monomers. Webster, O. Science 1991, 251, 887. Unfortunately, these types of polymerizations require the complete absence of water from the reaction mixture, and therefore, can not be conducted as emulsions or suspensions in water. Living polymerizations based on free radical mechanisms are only a recent development, as the ability of propagating radicals to react with one another (through combination, chain transfer or disproportionation for example) made reduction of termination reactions very difficult. Two current methods of "living"/controlled radical polymerization are TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl radical) mediated polymerizations [U.S. Pat. Nos. 4,581,429 and 5,322,912] and atom transfer radical polymerization (ATRP).[U.S. Pat. No. 5,763,548; Ser. Nos.: 09/034,187, 08/559,309, 08/677,828, 08/940,985, 09/018,554]
Nearly all controlled/"living" radical polymerizations, however, have been confined to bulk or solution polymerization. Although some polymerizations have been conducted in water, either as homogeneous (Coca, S., et al. J. Polym. Sci., Polym. Chem. Ed. 1998, 36, 1417) or biphasic mixtures, (Nishikawa, T. et al., Macromolecules 1997, 30, 2244.) the preparation of well-defined polymers by emulsion or suspension systems has not been reported. Both seeded (Bon, S. A. F. et al, Macromolecules 1997, 30, 324) and unseeded (Marestin, C. et al., Macromolecules 1998, 31, 4041.) emulsion polymerizations of styrene have been conducted with TEMPO-mediated polymerization systems, but TEMPO mediated polymerizations are generally successful only for polymerizations which predominately contain styrene, or derivatives of styrene, as monomers. Seeded emulsions are not desirable as the use of high molecular weight polymer as the seed will not conform to the same molecular weight of the final polymer and hence, introduce broader molecular weight distributions or differences in functionality. Also, TEMPO mediated polymerizations in general require the use of temperatures greater than 100.degree. C., requiring the use of specialized equipment to conduct the polymerizations to high conversions. The use of ATRP in emulsions (Makino, T. et al., Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1998, 39(1), 288.) or suspensions (Granel, C. et al., Macromolecules 1996, 29, 8576.) has been reported but the resulting polymers were not well-defined as evidenced by broad molecular weight distributions and the theoretical molecular weights that did not correspond with predicted values (DP.sub.n =.DELTA.[M]/[I].sub.o) nor did the molecular weights increase with conversion in a linear fashion. Other work previously reported to produce aqueous block copolymer dispersions and emulsions using azo macroinitiators (U.S. Pat. No. 5,741,845) also produces products with broad molecular weight distributions and no control over polymer architecture and end group functionalization.
Polymerizations carried out in heterogeneous systems, such as suspensions or emulsions in inorganic media are desirable for many reasons. By polymerizing organic monomers in water or CO.sub.2 as the reaction medium, the use of volatile organic compounds in preparation, processing or application of the polymer can be reduced or eliminated; the continuous phase can better dissipate heat generated by the polymerization, allowing for the monomers to be polymerized in bulk, and may enhance the polymerization rate of some monomers; polymerization in an emulsion or suspension can result in a reduction of viscosity of the system at high monomer to polymer conversion and can produce products of direct interest in various applications.